Unilever bought a majority stake in Chinese water-purification company Qinyuan Group Co., the Anglo-Dutch company's first acquisition in China in a decade and moving into a market with high demand for clean water.

The purchase enables Unilever to expand its Pureit purification brand through Qinyuan's distribution network across hundreds of cities in China, said Unilever Chief Executive Paul Polman. The consumer-goods company, known...